DC/AC converters are in widespread use. A typical field of use for DC/AC converters, in particular high power DC/AC converters, are isolated networks, which are operated at AC voltages and whose power supply is a DC power source.
The high level structure of a common DC/AC converter 2 is shown in FIG. 1. The DC/AC converter 2 comprises a DC/DC conversion stage 4 and a DC/AC conversion stage 6. The DC/DC conversion stage 4 allows for a voltage conversion between a first DC voltage VDC,Prim and a second DC voltage VDC,Sec. The DC/AC conversion stage 6 allows for a voltage conversion between the second DC voltage VDC,Sec and an AC voltage VAC. While the DC/AC conversion stage 6 is a necessary portion of the DC/AC converter 2, the provision of the DC/DC conversion stage is dependent on the particular requirements for the DC/AC converter 2. Such a DC/DC conversion stage 4 may be provided, because it allows for a galvanic isolation between the DC end and the AC end of the DC/AC converter. It also allows for the provision of a DC voltage to the DC/AC conversion stage 6 that may be different from VDC,Prim supplied by a DC power source.
FIG. 2 shows a previous approach DC/AC conversion stage 6 having three AC phases. A DC voltage, in the present case denoted VDC,Sec, is present at the DC side of the DC/AC conversion stage 6. At its AC side, the DC/AC conversion stage 6 has three AC terminals 80, 82 and 84, each of which is associated with one of the three phases of the AC voltage VAC. Besides, the DC/AC conversion stage 6 has a neutral terminal 86, which provides the voltage reference, also denoted neutral phase, to the three AC terminals 80, 82 and 84. The neutral terminal is connected to ground. The neutral terminal 86 may be the center point of an AC star configuration having the three AC phases present at the AC terminals 80, 82 and 84. The DC/AC conversion stage 6 further comprises four half bridge converters 70, 72, 74 and 76, all of which are coupled to the DC side terminals of the DC/AC conversion stage 6. Three of these half bridge converters, namely the converters 70, 72 and 74, are associated with the three phases of the AC voltage, while the fourth converter 76 is associated with the neutral phase of the AC voltage system. These four half bridge converters are coupled to the three AC terminals 80, 82 and 84 and to the neutral terminal 86 via a filter 78, wherein the filter 78 is provided for conditioning the AC voltage system.
FIG. 3 shows a previous approach implementation of the DC/DC conversion stage 4 having a pair of first side terminals 10, across which the DC voltage VDC,Prim is coupled, and a pair of second side terminals 12, across which the DC voltage VDC,sec is coupled. The pair of first side terminals are coupled to a first side converter circuit 20, which in turn is coupled to a transformer circuit 140, which in turn in coupled to a second side converter circuit 130, which in turn is coupled to the second side terminals 12. Each of the first side converter circuit 20 and the second side converter circuit 130 comprises an H bridge circuit, whose switches are controlled in such a way that a desired power transfer from the first side terminals to the second side terminals or from the second side terminals to the first side terminals takes place.
Previous approach DC/AC converters, as described above, have the disadvantage that they deviate from their desired electric behaviour in an unacceptable manner when changes in the operating conditions occur. Such changes may consist of varying electric loads applied to the DC side or to the AC side. Such changes can also consist of a change of power flow direction between the DC side and the AC side. These deviations from the desired behaviour can occur both in stationary states as well as relate to the dynamic response of the DC/AC converter to operating condition gradients.
Therefore, the problem underlying the present invention is to provide a method of controlling a DC/AC converter that allows for a reduced sensitivity of the conversion system to changes in operating conditions, i.e. to allow an acceptable system behaviour over a wider range of operating conditions.